NI 6215 - Providing Current though Analog Out

[AMMA]

I have a pressure sensor that I want to provide an excitation voltage with.  It takes max 2.1mA (typical 1.6mA) at 3.3VDC based on the datasheet

I noticed that that the spec sheet for the NI USB 6215 is 2mA for current.

I thought I would be fine, but everything I hook up AO0 to try to power the pressure sensor, the voltage would drop down to 0.274V.  I thought initially I had a bad connection, but if I remove sensor excitation connection it works again (Albiet I have to reset the DAQ)  

Any thoughts?  Can I use the 6215 to deliver 2mA at 3.3VDC?  

I tried setting up the 6215 to provide current, but it doesn't let me create a DAQmx task for current using this guy.  

Thanks

[Craig_]

Based on the specs it should be able to do this.

It may be worth connecting a 5V power supply to the +5V pins to provide additional voltage to the unit. This is described on page 3-3 of the manual in the section titled +5 V Power as an Output. It only talks about using this in respect to providing additional current to the Digital outputs but it may help the AO as well. http://www.ni.com/pdf/manuals/371931f.pdf

Otherwise you could just provide the 3.3V from an external source and measure the voltage across the sens and ground wires of the sensor.

[AMMA]

To provide power you are suggesting I use the digital lines instead?  

I'm trying to avoid using an external supply because it would help to keep this project have an extremely small footprint (basically an extra wire going to the DAQ).

I currently have tested it in a configuration with an external power supply, but it isn't my preferred approach.

[Craig_]

Actually it looks like the 6215 has 3.3V digital logic levels, you definitely could try outputting a true to one of the digital ports for powering the sensor as they are specced to source more current than the AO.

Tell me how it goes!

[AMMA]

I tested it out and probed the Voltage on a Digital Line set to high, and its 5V.  

I didn't see a means to set the digital line to 3.3V in Measurement and Automatino. 

The max Voltage my chip can take is 3.6V, so I dopnt want to fry it.

Can I set the D-HI value?

[Craig_]

Alright, I definitely was reading the specifications wrong.

This PDF: http://www.ni.com/pdf/manuals/371932f.pdf

Shows this picture in reference to the DO voltage levels for these USB devices:

I think these should be our conclusions:

• AO cannot source enough current for your sensor and the voltage isn't holding
• You cannot set the DO voltage output level (this is typical)
• You cannot count on the current draw from the DO to bring the voltage down to 3.3V reliably.

The external power supply is definitely the way to go!

[johnsold]

Other ideas:

1. Use an external LDO regulator from the 5 V output to 3.3 V. For example LT3008.

2. Use both AO lines in parallel. Connect a small resistor (~5 ohms?) in series with each line to allow for any slight mismatch in the output levels. Write the vlaues to both lines at the same time.

Lynn

[AMMA]

http://www.ni.com/pdf/manuals/371932f.pdf

http://sine.ni.com/ds/app/doc/p/id/ds-9/lang/en

From that document and the spec sheet we can see that it clearly lists "2mA" under Analog Output... I'm still confused why I can't draw that current from the AO.  Is it discussing this current draw within a different context?  At the very least I'd like to  understand why I can't do it so I can make sure I'm understanding the specs in these documents correctly.  

Analog Output

Output coupling		DC
Output impedance		0.2 Ω
Output current drive		±2 mA
Overdrive protection		±30 V
Overdrive current		2.4 mA

[Craig_]

Connect your DMM across the AO lines in current mode and see what you get.

Also, try connecting the DMM in current mode in series with your sensor to see how much current the sensor is actually using.

Both of Lynn's recommendations are great as well.

[AMMA]

So the plot thickens...I'm controlling everything through measurement & automation right now.

- When I put the DMM and directly measure the current from the AO, when I set it to 3.3V, I draw 2.33mA, which is in line with the spec sheet (It can overdrive to 2.3 mA).

- Using the particular chip I have, When powering it with the power supply at 3.3VDC, I'm drawing 1.89 mA.

- When I connect the Chip to AO0, It drops to 0.27V Again

- So then I looked at using both AO ports together (I didn't place a 5Ohm resistor, I just shorted them using 1/2 foot wire) and I was able to draw about double the current with the DMM(4.5 mA).  At this point, the chip started to function correctly, and the voltage held at 3.3V.  

-Then I measured the current draw with another DMM, and got 1.90mA.  

Sounds like it has some issue providing 1.9mA of current at 3.3V from one AO port with a load.  Is there something in the spec sheets that would indicate this?